Digital tomosynthesis is an imaging technique that provides three-dimensional (3D) patient images, reconstructed from a series of multiple 2D images taken over a succession of angles of the x-ray source relative to the detector. Acquisition of the 2D projection images used for tomosynthesis employs a large-area digital detector, such as a digital detector that is typically used for conventional single projection radiography.
In conventional tomosynthesis, a single X-ray source is moved along a generally linear scan path for generating multiple exposures. The set of projection image data that is acquired with tomosynthesis, by comparison, with full volume image information obtained using computed tomography (CT) or full volume imaging apparatus, is somewhat less detailed with respect to full volume information, but still allows sufficient 3D visualization for a number of diagnostic functions, at reduced exposures when compared against CT modalities. The projection images obtained for tomosynthesis, when digitally processed, yield a 3D image similar to computed tomography but with a more limited spatial resolution in the depth direction. Depth data is reconstructed from the captured projections in the form of a number of slices through the patient anatomy, with the best spatial resolution in the slices parallel to the detector plane. A consequence of limited angular scanning for tomosynthesis imaging is that the depth resolution is characteristically lower than a standard CT, but the in-plane resolution can be much higher due to the use of high resolution x-ray detectors in tomosynthesis.
The various types of tomosynthesis and tomographic imaging obtain depth information by virtue of the change of relative angle between the x-ray source and the subject for each projection image. This change is generally accomplished by movement of the x-ray source relative to the subject patient, with or without corresponding movement of the detector. The scan path of the x-ray source or detector for tomosynthesis can be linear, arcuate, or can have a planar circular arrangement. In applications where the detector is fixed, one or more movable sources may be displaced in a motion direction to vary the angle at which radiation is directed through the subject. Where an array of x-ray sources is used, the relative angle between source and detector is effectively changed by energizing successive elements of the array synchronously with image capture. Alternately, the source can remain stationary and the detector moved to different positions relative to the source and patient. Since the image is digitally generated and represented, various processing techniques can be used to generate and present a series of slices at different depths and with different thicknesses reconstructed from the same image acquisition.
Conventional tomosynthesis acquisition consists of a number of projections of X-ray exposures covering an angular range less than 180 degrees, typically 20 to 50 degrees. The patient typically stands near the detector plane during the tomosynthesis scan. The number of projections for a single wallstand scan can range from about 30 to 60. The sweep angle is the angle from the first to the final projection focal spot with respect to the focal plane.
During the scan sequence, the X-ray source is moved to different focal spot positions and a projection image for the tomosynthesis series is acquired at each position. After tomosynthesis acquisition, the digital images acquired at the detector are reconstructed into multiple image slices using a computerized reconstruction algorithm, and then viewed from an aspect in parallel to the flat panel detector face. The digital flat panel detectors developed for tomosynthesis provide rapid response, excellent dynamic range, and good quality digital images for input to the reconstruction software.
Viewing reconstructed slices is the customary and primary method of visualizing digital tomosynthesis imaging data. However, a common complication of the process of slice reconstruction is reconstruction artifacts. These artifacts result from a number of causes, particularly from an insufficient number of projections, and also due to limited angle of data acquisition and ill-posed nature of the limited view reconstruction problem.
One aspect of conventional tomosynthesis imaging is the requirement for obtaining multiple 2D projection images of the patient in order to allow 3D reconstruction. The requirement to obtain numerous images adds to the dosage levels that this imaging modality entails. In addition, this requirement also makes patient motion a problem, since some amount of motion is unavoidable for any practical exposure duration over which multiple images are obtained. Attempts to reduce the number of projection images acquired result in view artifacts in the reconstructed images, including aliasing, ripple, and other undesirable effects.
With the advent of more portable apparatus, the use of tomosynthesis imaging in clinical environments is expected to increase. These environments, however, can present challenges for tomosynthesis imaging, since patient movement during the image acquisition sequence is more likely in the ICU (intensive care unit) and clinical setting and can cause difficulties with obtaining the needed series of images.
Thus, it can be appreciated that methods that reduce the number of 2D projection images that are needed for tomosynthesis can help to reduce dosage requirements and can also help to remedy imaging problems related to patient motion.